The present invention relates in general to an apparatus for measuring the flow velocity of various fluids, and more particularly, to an ultrasonic flowmeter of the type employing externally arranged transducers and internally arranged acoustic reflectors for the nonintrusive measurement of fluid velocity within a conduit such as a pipe, channel, annular region, and the like.
Fluid flow velocity may be determined from the measurement of the time taken for acoustic pulses to traverse a predetermined path in the fluid respectively in the direction of flow and opposite to the direction of flow. These time measurements may be carried out using two pairs of transducers, each pair comprising a transmitter and a receiver of, for example, ultrasonic acoustic pulses. The transducers are arranged so that the ultrasonic acoustic pulses travel along equal length paths in opposite directions within the fluid whose velocity is being determined.
One of the many applications for which ultrasonic methods have unique capabilities is in the measurement of water flow in the annular downcomer region of nuclear power plant steam generators. The downcomer annulus is that space between the heavy steel outer shell and the thinner steel wrapper which surrounds the tube bundle. Typical radial dimensions of the downcomer are 3 inches for the shell, 2.5 inches for the annulus, and 0.38 inches for the wrapper.
It has been recognized that downcomer fluid flow is an important parameter in the controlled operation of recirculating steam generators, yet, no suitable instrument for flow measurement in the downcomer annulus is presently available. As a result of the downcomer construction, the ultrasonic transducers must be arranged on the outside of the outer shell and on the same side of the flow region. The refraction of the acoustic pulses at the steel-water interface restricts the path angle of the acoustic pulses in the fluid to a direction more nearly perpendicular to the fluid flow direction than is desired for good sensitivity. This applies to the travel time difference method of determining the velocity of fluid flow in which the flow component v cos .theta. is sensed, where v is the flow velocity and .theta. is the angle between the flow vector and the acoustic path. It can therefore be appreciated that as the angle between the flow vector and the acoustic path increases, there is a corresponding decrease in the sensitivity of the ultrasonic flowmeter. This is particularly a disadvantage when measuring water flow in the annular downcomer region of nuclear power plant steam generators.
It has been found that even if shear acoustic pulses are propogated in the steel outer shell, the minimum value obtainable of .theta. is about 65.degree. in room temperature water. At the typical operating temperature of 600.degree. F. for nuclear power plant steam generators, the reduced sound velocity in water limits the minimum value obtainable of .theta. to about 75.degree.. Thus, the change of .theta. with temperature, the internal reflections in the steel outer shell, and the necessity to couple transducers at an angle to the outer shell are factors that complicate the design of the ultrasonic flowmeter and seriously reduces its potential accuracy. Accordingly, it can be appreciated that there is an unsolved need for an ultrasonic flowmeter for the nonintrusive measurement of water flow in conduits, such as in the annular downcomer region of nuclear power plant steam generators which has good sensitivity by minimizing the angle between the flow vector and the acoustic path.